Can someone explain the math/rationale behind this behavior?
Let see how waves is generated.

It takes your x coordinate (X will be go down like in your view), then multiples by scale value in your case it is 1:

Numbers 1,2,3 ets will be white, 0.5, 1.5, 2.5 ets - dark (real algorithm uses Sin function to convert this number to lightness, but I get this because it simpler to view):



Usually, y coordinate uses too, but you separate coordinates and get only first one, so I set Y coordinate as scale because it simpler to view.

Now let see, that happens if i set Scale to 2:



As you see, waves gets smaller. If i make the scale bigger I will not see waves, just because my grid haven't enough resolution for this. So instead I try to put Y coordinate as scale to see that it will do:



So, you see this effect because scale is not regular and waves gets smaller and smaller while scale gets bigger across y-coordinate.